A motor can be regarded as a multi-functional component which includes a rotor, a shaft, a bearing, a stator, and the like obtained by highly precisely processing various materials such as steel, non-ferrous metal, and polymer and which converts electric energy into mechanic energy by the combination thereof. In recent motors, a permanent magnet type motor which uses a magnet capable of attracting or repelling other magnetic materials and of permanently generating a static magnetic field without the external energy is widely used. From the viewpoint of physics, the magnet is different from other magnetic materials in that an effective magnetization remains even after canceling an external magnetic field, a magnetization inversion (demagnetization) eventually occurs upon being applied with a heat or a comparatively large inverse magnetic field, and then a magnetization reduces with the demagnetization. An important characteristic value of the magnet is an energy density (BH) max. The energy density shows potential energy of the magnet by the unit of volume.
Incidentally, the high performance of the strong attracting or repelling capability of the magnet is not always dependent on the type of the motor. However, in Non-patent document 1, on the basis of a relationship between a residual magnetic flux density Br corresponding to one of basic characteristics of the magnet and a motor constant KJ (KJ is a ratio between an output torque KT and a square root √R of a resistance loss) corresponding to an index of a motor performance, it is described that an increase in energy density (BH) max of the magnet induces the higher torque density in the small-sized motor using a ring magnet, which is a target of the invention, in the state where a motor diameter, a rotor diameter, a gap, a soft magnetic material, a magnet dimension, and the like are fixed.
However, since a stator iron core of the motor is provided with teeth forming a part of a magnetic circuit and a slot accommodating coiled wires, the permeance changes with the rotation. For this reason, the increase in energy density (BH) max of the magnet increases a torque pulsation, that is, a cogging torque. The increase in cogging torque causes harmful influences such as the disturbance of a smooth rotation of the motor, the increase in vibration or noise of the motor, and the deterioration in rotation control performance.
In order to avoid such harmful influences, many studies on the cogging torque reduction of the motor have been carried out in the past.
First, regarding the magnetic pole having an even thickness in a magnetization direction, the uneven thickness of the magnet is considered. For example, Non-patent Document 2 describes a small-sized motor including magnetic pole 1 having an uneven thickness, stator iron core 2, stator iron core slot 3, and stator iron core teeth 4 as shown in FIG. 11A. That is, in Non-patent Document 2, it is described that the cogging torque becomes minimal when the surface mounted permanent magnet synchronous motor (SPMSM) has the 12-pole-18-slot uneven-thickness magnetic pole having a configuration in which a residual magnetization Br is 1.2 T, a maximum thickness at the magnetic pole center is 3 mm, and a minimum thickness at both magnetic pole ends is 1.5 mm. Additionally, in this case, the thickness is uneven from the outer diameter side of the magnetic pole. However, it is known that the cogging torque can be reduced even in the magnetic pole of which the thickness is uneven from the inner diameter side of the magnetic pole.
In addition, in Non-patent Document 2, in order to minimize the cogging torque by means of the uneven thickness of the magnetic pole shown in FIG. 11A, the minimum thickness of both magnetic pole ends is required to have the uneven thickness so that the thickness is about a half of the maximum thickness of the magnetic pole center. Accordingly, if the thickness of the magnetic pole, that is, the magnetization direction (thickness) becomes thinner, sufficient advantage cannot be obtained even when the thickness of the magnetic pole becomes uneven so as to minimize the cogging torque. Generally, since the magnetic pole is mechanically weak, the processing thereof becomes difficult.
Meanwhile, regarding the magnetic pole of which the thickness is thin in the magnetization direction, there are known a method of skewing the magnetic pole of FIG. 11B disclosed in Non-patent Document 3 or a method of continuously removing a magnetic pole area between magnetic poles of FIG. 11C disclosed in Non-patent Document 4.
According to the summary of the known techniques described above, the magnetic pole end of the thick magnetic pole is thinned by about a half so as to broaden a gap between itself and the stator iron core or the area between the magnetic poles of the thin magnetic pole is removed. Accordingly, the amount of the static magnetic field Ms, generated from the magnetic pole and flowing into the stator iron core in the form of the magnetic flux Φ, is suppressed due to the increase in magnetic resistance. As a result, in these methods, the torque density decreases by 10 to 15% in general due to the reduction in cogging torque. Accordingly, the cogging torque reduction methods using the known techniques shown in FIGS. 11A, 11B, and 11C are contrary to the technique in which the increase in energy density (BH) max of the magnet induces the increase in torque density of the motor.
Meanwhile, in Non-patent Document 5, a cogging torque reduction method of the motor is reported. In Non-patent Document 5, using a rare-earth/iron-based sintered magnet of which the thickness is thin in the magnetization direction to be 1.2 mm and the residual magnetization Mr has the high energy density of 1 T, the cogging torque is reduced by the method shown in FIGS. 11A, 11B, and 11C in which the thickness in the magnetization direction or the magnetic pole area does not decrease. That is, as shown in FIGS. 12A to 12D, a so-called Halbach Cylinder is shown of which each magnetic pole is formed into two to five divided sections, and the anisotropic direction (magnetization easy axis direction) for each divided section is adjusted stepwise. Here, in the drawing, the suffixes (2) to (5) of magnetic pole 1 indicate the number of two to five divided sections of magnetic pole 1. In addition, the direction indicated by the arrow of each divided section indicates the anisotropic direction (the magnetization easy axis).
When the 12-pole-18-slot motor is manufactured by adopting the magnetic pole having the above-described configuration, number N of the divided magnetic pole sections and the cogging torque Tcog satisfies the exponential approximation as Tcog=61.753 exp (−0.1451×N). That is, it is suggested to be ideal that, when Mθ denotes a magnetization vector angle formed between magnetization vector M at an arbitrary mechanical angle φ and the circumferential tangential line of the magnetic pole, a regularly continuous change with high accuracy is taken between the magnetic poles. However, in the rare-earth/iron sintered magnet having a thickness of 1.2 mm and a high energy density comparable to a residual magnetization Mr of 1 T, it is difficult to prepare plural magnetic pole sections having different anisotropic directions, to arrange minutely and regularly the magnetic pole divided sections, and to constitute the rotor by configuring a plurality of magnetic poles with high dimensional precision. For this reason, it is very difficult to manufacture a multi-pole rotor having an integral multiple of the magnetic poles and the small-sized motor adopting the multi-pole rotor. In addition, it is easily supposed that compatibility with economical efficiency are insufficient.
A magnetically isotropic magnet can be freely magnetized in a direction of a magnetization field and in any direction in accordance with a magnetic field strength distribution. For this reason, it is possible to have a magnetization pattern indicated by the circular arc arrow in a magnetic pole 1 of FIG. 13 by means of a shape of a magnetization yoke and an optimization of a magnetomotive force. Accordingly, it is possible easily to adjust a gap magnetic flux density distribution between a magnetic pole and a stator iron core to a sine wavelength. Thus, the cogging torque reduction in the small-sized motor such as the SPMSM can easily be carried out compared with the case where a thin magnetic pole is formed of a magnetically anisotropic magnet material.
A study on the isotropic rare-earth magnet material has been started by R. W. Lee (see Non-patent Document 11) and others. In Non-patent Document 11, an isotropic Nd2Fe14B-based bond magnet having the energy density (BH) max of 72 kJ/m3 can be formed when a rapidly-solidified ribbon having the energy density (BH) max of 111 kJ/m3 is fixed by a resin. Since then, a study on the isotropic rare-earth magnet material mainly obtained by the rapid solidification of the rare-earth-iron-based molten alloy has been actively carried out from the late in 1980s up to now. For example, Nd2Fe14B-base, Sm2Fe17N3-base, and their nanocomposite magnet material with αFe-base, FeB-base, and Fe3B-base using an exchange bonding based on a microscopic structure come to be used in industry. Also, in addition to an isotropic magnet material obtained by the micro control of various alloy structures, an isotropic magnet material having different powder form is widely used in industry. For example, see Non-patent Documents 6 to 10. Particularly, in Non-patent Document 10, H. A. Davies and others have proposed a material having an isotropy and an energy density (BH) max of 220 kJ/m3.
However, the energy density (BH) max of the isotropic magnet material which can be used in industry is 134 kJ/m3 at best. In the application of the magnet motor represented as a small-sized motor having a power of 50 W or less, generally, the energy density (BH) max of the isotropic Nd2Fe14B-based bond magnet is approximately 80 kJ/m3 or less. That is, although twenty years have passed since the time when the isotropic Nd2Fe14B-based bond magnet having the energy density (BH) max of 72 kJ/m3 is formed from the ribbon having the energy density (BH) max of 111 kJ/m3 by R. W. Lee and others in 1985, the improved energy density (BH) max is smaller than 10 kJ/m3.
Accordingly, the energy density cannot be improved in accordance with the slow development of the isotropic magnet material. Also, the increase in torque density of the motor which is a target of the invention cannot be expected.
Meanwhile, the energy density (BH) max generally increases when the isotropic magnet is exchanged to the anisotropic magnet. For this reason, in the small-sized motor, the higher torque density can be obtained, but the cogging torque increases. In addition, in the existing radial anisotropic ring magnet, if the inner/outer diameter decreases, leaked magnetic flux increases even when external magnetic field Hex is repelled by a center core of a ring cavity so as to generate a radially oriented magnetic field. Thus, energy density (BH) max deteriorates. In particular, in a diameter of 25 mm or less, the tendency becomes strong.
As an isotropic rare-earth/iron-based magnetic material related to the present invention, for example, there is RD-Sm2Fe17N3 of Non-patent Document 12 or HDDR-Nd2Fe14B of Non-patent Document 13.